Histochemical Studies of Jelly Coat of Marthasterias glacialis (Echinodermata, Asteroidea) Oocytes

Mechanical forces imposed on echinoid eggs during spawning: mitigation of forces by fibrous networks within egg extracellular layers

Many echinoderms spawn large numbers of eggs in rapid spawning bouts directly into the water column. During spawning, the eggs pass from the gonad along a narrow oviduct and through a rigid gonopore before reaching the water. As the eggs traverse this pathway, they are exposed to shear stress induced by the development of a velocity gradient within the fluid formed by the eggs. In some species, the diameter of the eggs is larger than the diameter of the gonopore. In these cases, the eggs also experience strain resulting from compression of the egg as it passes through the relatively small gonopore. The magnitude of shear stress experienced by eggs differs among species depending upon the viscosity of the eggs. The degree of strain experienced by eggs differs among species depending upon the relative sizes of the egg and gonopore and also changes within species as they grow. Recent evidence suggests that these forces have the propensity to damage eggs, thereby reducing their fertilizability, or to destroy them. Experimental evidence shows that the jelly coat around the eggs can protect them from shear stresses and reduce the strain imposed on them under a compressive force. Echinoderm eggs are surrounded by a jelly coat that has a composite structure of irregularly arranged fibrous networks embedded in a jelly matrix. These fibrous networks have characteristics that are similar to those of engineered and biological materials that are known to reduce the transmission of mechanical forces to other structures. Given this similarity to other materials, three possible mechanisms for the action of the jelly coat may be identified. First, the apparent viscosity of the eggs of echinoids declines as the rate at which they are sheared during spawning increases. This reduction in viscosity with increasing shear rate (shear-thinning) may, in part, be due to the structure of the jelly coat and its resultant non-Newtonian nature. Second, experimental evidence indicates that the jelly coat preferentially deforms under a compressive load, reducing the transmission of that load to the egg. Third, the jelly (but not the fibers in the coating) may deform in a nearly viscous manner. In this case, the fibers may serve to provide an elastic ‘backbone’ to the layer and remain in place to stiffen the outer layer of the egg. The composite structure of the jelly coat and the morphology of the fibrous network are likely to be critical to all these mechanisms.

Shear stress experienced by echinoderm eggs in the oviduct during spawning: potential role in the evolution of egg properties

Shear stresses experienced by eggs in the oviduct of the echinoid Arbacia punctulata during spawning were calculated using engineering equations that describe laminar flow through pipes. Shear stresses in the oviduct ranged from 0 to 58.7 Pa. Two properties of eggs were identified that have the potential either to minimize the shear stress in the oviduct or to reduce the damage experienced by eggs exposed to high shear stress. These properties are the viscosity of the eggs and the presence of extracellular layers on eggs of A. punctulata. The viscosity of eggs decreases with increasing shear rates, which reduces the magnitude of shear stress experienced in the oviduct, while the extracellular layers mitigate the effect of shear stress on the eggs. Eggs with intact extracellular layers were damaged less frequently than were those with the extracellular layers removed. The results of this research indicate that physical stresses may be important selective factors in the evolution of gamete properties.

The echinoderm surface and its role in preventing microfouling

Antifouling is a property of the epidermis in echinoderms. There is neither production of biocides that act at any distance from the surface nor is the sloughing rate of the entire surface capable of explaining the observed antifouling capability. As with many invertebrates, the epidermis in echinoderms is overlain by thin surface coats, often termed the cuticle. The outermost coat has attenuated fibrils radiating outwards from the underlying cuticle. As these fibrils are the "real"; surface of the echinoderm, this is the level at which any antifouling defense must operate. It is suggested that their function is primarily antifouling. The cuticle contains chondroitin sulphate proteoglycan molecules and is negatively charged. The cuticle appears to be a highly extended glycocalyx. It is suggested that the primitive function of cellular glycocalyces is to modulate adhesive interactions at the cell or organismal surface.